Two forms of the DNA of murine leukemia viruses will be analyzed. They are the unintegrated closed circular DNA molecules and the integrated viral DNA of both rat cells infected with Moloney murine leukemia virus and various tissues of the AKR mouse. A deletion that has been found in a smaller closed circular molecule will be localized both by restriction endonuclease cleavage and by S1 nuclease digestion of DNA:RNA hybrid molecules. The biological activity of the closed circular DNA's will be tested by transfection experiments. The cellular sites of integration of viral DNA of both infected rat cells and tissues of AKR mice will be analyzed by restriction endonuclease cleavage, gel electrophoresis and the Southern transfer technique. Determinations of the copy number and of the similarities and differences among the different proviral sites will be made.